Oxidation of low density lipoprotein (LDL) may be of pivotal importance in atherogenesis, but the mechanisms that promote oxidation in vivo remain poorly understood. We have explored the possibility that one pathway involves myeloperoxidase, a heme protein secreted by phagocytes. Myeloperoxidase is the only human enzyme known to generate HOCl, a potent oxidizing agent, at physiological halide concentrations. LDL exposed to the complete myeloperoxidase-H2O2-Cl system underwent chlorination of its protein tyrosyl residues as demonstrated by mass spectrometric measurement of chloro-tyrosine. Treatment of LDL with reagent HOCl resulted in 3-chlorotyrosine formation, implicating HOCl as an intermediate in the enzymatic reaction pathway. In contrast, 3-chlorotyrosine was undetectable in LDL oxidized by hydroxyl radical, copper, iron, hemin, glucose, peroxynitrite, horseradish peroxidase, lactoperoxidase or lipoxygenase. These results indicate that 3-chlorotyrosine is a specif ic marker for LDL oxidation by myeloperoxidase. The detection of 3-chlorotyrosine in human atherosclerotic lesions indicates that halogenation reactions catalyzed by the myeloperoxidase system of phagocytes constitute one pathway for protein oxidation in vivo. These findings raise the possibility that the myeloperoxidase-H2-C2-Cl system plays a critical role in converting LDL into an atherogenic form.